The present invention relates to turbine shroud designs, in particular for low-pressure turbines of turbine engines.
It is common to incorporate an expanded section at the outer ends of turbine blades, these expanded ends interlocking to form a ring or shroud, which assists in support of the blades, in sealing and in reducing vibrations. However, the shroud itself adds considerable weight.
In order to reduce the weight of the shroud, pockets can be introduced, as shown for example in U.S. Pat. No. 6,491,498. These are effective in reducing weight but do not take sufficiently into account dynamic loads.
Such an outer shroud section 1 is shown in FIG. 1, as a top view in the radial direction. The outer shroud 1 comprises two sealing fins 2 and 3 extending in the circumferential direction and arranged parallel to one another as shown at the top and at the bottom of FIG. 1. Between the two fins 2 and 3 on the left-hand side and on the right, “Z-notches” 4 and 5, i.e., interlocking Z-shaped boundary walls, having edges 8, are arranged so that they frame the pocket 6 mentioned above. This pocket is effective in reducing weight but does not take sufficiently into account dynamic load. Therefore the outer shroud will bend around a bending axis A. Additionally a rib 7 extending substantially in the direction of the bending axis A is arranged in the pocket 6. Such a reinforcement does not help to avoid or reduce the above-mentioned drawback.
The object of the invention is to reduce the bending tendency of the outer shrouds of turbine blades.
Thus, according to the invention, there is provided a turbine blade having an outer shroud section at the radially outer end, the blade intersecting the shroud section, wherein the shroud section has at least one reinforcing rib extending along its outer face, wherein the rib extends substantially in the direction of the principal axis of inertia.
The invention aims to reduce stress levels at the outer shroud (OS), especially at Z-notch areas by placing OS reinforcement ribs on the airfoil Imax bending axis. A more efficient use of OS reinforcement reduces the shroud mass and consequently the blade weight and overall mechanical loads. The approach is particularly suited for High-Speed LPTs with large shrouds (low airfoil count) where the centrifugal loads dominate, causing high radial bending stresses.
The transverse rib or ribs at the top of the shroud, more or less perpendicular to the airfoil bending axis, ensure a high resistance against radial folding. Hence stresses from bending are reduced. This solution proves very effective in the case of a flat outer annulus profile, where the structure is particularly prone to bending. This structure advantageously reduces mechanical loading and consequently weight.
This configuration affords the maximum reinforcement for a given mass. There can be two, or three or more, evenly spaced ribs. The shroud may further have one, or preferably two, circumferentially extending sealing fins, also on the outer face of the shroud, for sealing with a soft part, e.g., a honeycomb, of the turbine housing. The ribs can then extend between these two fins or fin walls, but having a lesser height.
The invention also contemplates a rotor, and an engine, using such blades.
For a better understanding of the invention, embodiments of it will now be described, by way of example, with reference to the accompanying drawings.